Brad DeLong population postings
I'd like to draw your attention to three new postings on population growth at Brad DeLong's web site. The first, Crude Life Expectancy Estimates leads off with a striking figure reminding us how radically different the times we live in are from all of those that have gone before. Given the scales used, the estimated average world life span is almost flat in the low 20s from 0 to 1800 AD and then shoots upward almost vertically, reaching the mid-60s in 2000. The idea that the natural life span for a man or woman is 60-70 years, would not have made sense to anyone before the last century.
The figures are also broken out by country and region in a second figure. I note that, while India and Africa appear to be at the bottom at any point in time, life spans in both have risen in this century from the area of 20 years to something on the order of 60 years - almost a three fold increase in life spans in a hundred years. I posted a few days ago on the economic stagnation and decline in sub-saharan Africa since 1975. DeLong's figures appear to be based on data points in 1950 and 2000 and would not show a decline since 1975 unless it was greater than the increase from 1950 to 1975. Russian life spans, which rose rapidly between 1900 and 1950, essentially flattens out between 1950 and 2000 - in this respect behaving differently from those in any other region.
You could skip the second posting, because it's gist is repeated at the start of the third. DeLong leads off with a figure showing the relation between income and population growth since 1820: "The Demographic Transition". Population growth rates rise with income up to somethign on the order of $1,100 (1n "1990" dollars) and then more or less levels off. Once incomes reach $4,000, population growth rates begin to decline.
The third posting, "Notes: In the Shadow of Malthus" contains the argument and the punchline. DeLong's income and growth figure leads him to conclude that a population with an income level of $660 would be growing at an annual rate of something like 0.2% or so. A chart of estimated world human population growth rates since 2,500 BC suggests that it wasn't until about 1800 and after that population growth rates exceeded this level. His conclusion, before 1800 humans lived on something like $660 a year. I don't have an intuitive feel for the dollar measures, so I have difficulty visualizing what this means. DeLong offers this assistance:
"What held back population growth? What keeps the numbers of the human race from growing at more than 0.2% even under the most favorable pre-industrial conditions? The conclusion seems inescapable: desperate poverty. For the overwhelming bulk of recorded history, population growth rates have been kept low by poverty so dire that women's fat reserves are so low that ovulation is a hit-or-miss affair, and by poverty so dire that nutritional deficits are so great as to seriously compromise immune systems' abilities to deal with the endemic disease pool."
P.S. (7 pm): DeLong follows up with this pointed and not unrelated post:
"The Good Things in Life" -"Wendell Berry comes out foursquare in favor of the good things in life: short life expectancy, chronic malnutrition, plague, and poverty..."